one is a celebration of breaking fast, and the other a celebration of the sacrifice and start of Hajj.. they change because they follow a lunar calendar not a solar one.. lunar calendar is 11 days shorter per year than solar hence it changes..
Asalamu Alaikum sis thank u so much...this non-muslim was asking me and I was lyk 1 Eid is a sort of thanksgiving cos ur thanking Allah for the blessed month of ramadan and the other is the sacrifical eid then she asked how do u kno when they are and I was like well one is straight after Ramadan and the other after Hajj then she was like well how do u know when they are an i was lyk erm erm erm erm I'll find that out